It is a constant endeavor to find improved techniques of placing visible or invisible identifying marks on an image. This is generally useful to establish ownership, origin and authenticity, and also to discourage those who might wish to purloin or misappropriate the work. Identifying marks are also useful to give evidence of unauthorized alteration or disclosure. All heretofore watermarking methods are concerned with imparting a single watermark. There are advantages in imparting a composite watermark of a multiplicity of watermarks and henceforth detecting one or more watermarks from the multiplicity.
Invisible marks are herein classified relative to the appearance of that mark to a human being with normal visual acuity. A mark on an image is classified as having an invisibility classification level of undetectable invisible if, when the image without the marking is displayed together with an image copy with the marking, the human being is equally likely to select either of these copies. An undetectable invisible mark is below or at the human being's just-noticeable difference. A mark on an image is classified as having an invisibility classification level of subliminally invisible if the mark is not distracting to the human being, although it is above the human being's just-noticeable difference. An image marking is classified as being marginally invisible if it does not cause the marked image to lose its usefulness or value because of the mark. An image marking is classified as being poorly invisible if the marking causes a reduction in the image's usefulness and/or value.
Presently, invisible markings of hard copy documents are used as a generally dependable method of establishing ownership and authenticity. These time-tested methods are also useful for marking a “soft copy” digitized image, also referred to herein as a digital image or image. A digital image is an abstraction of a physical image that has been scanned and stored in a computer's memory as rectangular arrays of numbers corresponding to that image's (one or more) color planes. Each array element corresponding to a very small area of the physical image is called a picture element, or pixel. The numeric value associated with each pixel for a monochrome image represents the magnitude of its average brightness on its single color (black and white) plane. For a color image, each pixel has values associated and representing the magnitudes or average brightness of its tristimulus color components representing its three color planes. Other image representations have more than three color components for each pixel. A different value is associated with each different one of the image's color planes.
Whenever reference is made herein to color planes, it is understood to include any number of color planes used by a particular image's digitizing technique to define the pixel's color characteristics. This includes the case when there is only a single plane defining a monochromatic image.
Generally, a digitized image is recognizable as an image to a viewer only when the individual pixels are displayed as dots of white or colored light on a display or as dots of black or colored inks or dyes on a hard copy. Pixels are normally spaced so closely as to be not resolvable by a human visual system. This results in the fusion of neighboring pixels by the human visual system into a representation of the original physical image. Image fusion by the human visual system makes invisible marking, or relatively invisible marking, of images possible. This property is fully exploited by the methods described here to both impart upon a digitized image an invisible watermark to a desired invisibility classification, and to subsequently demonstrate its existence. The imparting and demonstrated detection of a robust invisible marking on digital images, herein called invisible watermarking, are a primary aspect of the present invention.